I have not been to "mainland China," but commonsense precautions apply concerning wearing jewelry everywhere. I never wear my diamond rings on trips. I have a plain gold band that I always wear while traveling. And I don't feel the need to wear expensive watches, relying on my good ol' Timexes instead.
Nice costume jewelry suffices for any of my travels, although sometimes I take pearls and/or some David Yurman pieces if I'm attending an out-of-town wedding or other dressy occasion.
If you're worried, why not make your mother-in-law happy by following her suggestion. You would gain a lot of points that way!
